The overall objective of this proposal is to investigate the role of platelet activating factor (PAF) in the lung vascular changes associated with chronic hypoxia-induced pulmonary hypertension (PH). We have previously shown that in fetal pulmonary vascular smooth muscle (PVSMC), under hypoxic conditions, PAF synthesis and PAF receptor (PAF-R) expression are high, and that this is down regulated at birth with oxygenation. Our main hypothesis is that in the hypoxic environment of the fetus, PAF plays a physiological role in maintaining high pulmonary vasomotor tone and may contribute to vascular growth; however, in chronic hypoxia, in utero or in the newborn period, there is excessive PAF-mediated signaling leading to PVSMC and adventitial fibroblast (AF) hyperplasia and vascular remodeling and abnormal vasoconstriction resulting in pulmonary hypertension. The specific aims are: 1. To demonstrate a role for PAF in chronic hypoxia-induced pulmonary hypertension in neonatal calves and in PAF-receptor knock-out and over-expressing transgenic mice. 2. To investigate the mechanisms by which PAF, in the presence of hypoxia, mediates its effects in pulmonary vascular smooth muscle and adventitial fibroblasts. Specifically, we will determine the signal transduction pathway by which PAF interactions with its receptor in hypoxia leads to SMC contraction; the mechanisms by which PAF stimulates PVSMC and AF proliferation in hypoxia and the mechanism of PAF-induced trans-differentiation of fibroblasts to myo-fibroblasts. These experiments will enable us to understand the mechanisms of PAF-mediated pulmonary vascular remodeling and vaso-constriction in chronic hypoxia. Results from our studies should increase our understanding of the cellular mechanisms by which PAF contributes to the pathogenesis of chronic hypoxia-induced PH. This should enable us to develop better methods of prevention and treatment of PH. [unreadable] [unreadable]